The demand for thin-film rotor evaporators is explained by the fact that they ensure "mild" conditions for processes of distillation, evaporation, concentration and vaporization making it possible to avoid decomposition and polymerization of the treated materials, especially thermally unstable ones.
Such apparatus is characterized by modest hydraulic friction and little or no hydrostatic head which permits vacuum treatment of materials (under a low pressure, at pressure of up to 1 mm Hg, i.e. lowering the temperature in the apparatus).
Furthermore, the residence time of the materials in these apparatus is relatively short (of the order of 5 to 40 sec.) as compared to other types of evaporators, which allows reduction of the degree of thermal treatment to a minimum.
Thin-film rotor evaporators are unique in that they combine high-intensity heat exchange and short time of residence of treated materials.
To this end, several rotor thin-film evaporators have been designed, wherein the liquid film on the heat-exchange surface of the apparatus is produced by rotor wipers rigidly secured on a power shaft and a 1-2 mm wide clearance is provided between the casing and the wipers.
The above-described apparatus is characterized by a limited surface of heat exchange because of its complicated design, production process, assembly and operation caused by the small clearance between the casing walls and the rotor wipers and by the necessity for dynamic balancing of the rotor to make allowance for the thermal strain of the rotor and the apparatus casing. The apparatus is sensitive to heat and liquid loads; the evaporator is most efficient with large liquid loads, that is separation of liquid drops from the steam flow becomes unreliable and insufficient with 25 to 50 percent of the initial amount of the starting material being discharged from the apparatus.
The problem of increasing the heat-exchange surface has been partially solved by another type of rotor thin-film evaporator wherein the liquid film on the apparatus heat-exchange surface is produced by rotor wipers pivotally mounted on the power shaft and sweeping the surface of the apparatus casing. The design of the apparatus permits building larger evaporators.
However, almost all other disadvantages of the known evaporator are present in this design. Besides, direct contact of the rotor wipers with the heat-exchange surface of the apparatus results in contamination of the material, apart from undesirable wear of the wipers and the casing.
The inner surface of the casing being rubbed by wipers must be thoroughly treated to make it a polished surface. The rotor wipers must be made of abrasion-resistant materials exhibiting good antifriction properties.
The required uniform fit of each wiper to the casing heat-exchange surface demands careful fabrication and assembly of the rotor.
The optimum thickness of the film produced by this method upon the heat-exchange surface is determined by a complex combination of factors, namely physical properties of the liquid on the one hand and the rotational velocity of the rotor, the weight of the wipers, their contact with the heat-exchange surface and the design features of the rotor, on the other hand. The best operating conditions are consequently achieved within a relatively narrow range of pressure upon the liquid by the wipers. If the optimum pressure is exceeded, the wipers may bare the heat-transfer surface (scratch off the film), whereas with lower pressure the liquid may drop through downwards.
The problem of increasing the heat-exchange surface (150-200%), simplification of the apparatus design, its fabrication and operation has been solved to a considerable degree by another design of a rotor thin-film evaporator wherein the liquid film upon the heat-exchange surface is produced without any mixing means. The liquid in this apparatus is distributed by centrifugal force by way of crimped drums mounted on the rotor shaft and perforated to discharge the liquid upon the heat-exchange surface of the apparatus casing.
The designs of the prior art apparatus do not exploit all ways to intensify heat exchange, namely: heat exchange intensification by making use of the entire surface of the apparatus casing over the height of the rotor; and heat exchange intensification by increasing the friction of steam and liquid (downflowing film). The latter shortcoming is accounted for by the fact that the steam formed on the heat exchanging surface is immediately evacuated through the perforations in the bulges of the drum crimps into their inner space.
The design of the device for uniform distribution of liquid over the inner surface of the bulges of the drum crimps is too complicated and bulky. It comprises a cylinder with a cogged base rigidly secured in the casing and disposed over the upper drum, a feeding device constituted as a sleeve secured to the rotor and mounted in the cylinder, its lower portion having radial pipes and a rimmed ring placed under the base of the fixed cylinder and secured in the upper part of the drum. Such design of the distributor involves two irrelevant stages of liquid redistribution, namely: intermediate distribution of the liquid over the inner surface of the fixed cylinder and distribution of the liquid on the drum ring. It should be taken into consideration that during these stages of distribution, the liquid may be carried away by a flow of steam (secondary capture) coming into contact with splashes and drops formed when the liquid is discharged from the feeder radial pipes and splashed from the surface of the fixed cylinder and the distributing ring of the drum.
A device for separating liquid drops from a steam-liquid flow positioned inside the apparatus casing over the upper drum, to be more exact in the annular clearance between the casing and the feeder sleeve, comprises a centrifugal separating device. This device comprises inclined wipers with bent upper edges secured to the rotor and a number of vertical plates secured rigidly and radially to the casing, a concentric ring being mounted over these plates. This device does not secure complete separation of liquid drops. The separation is single-staged and the steam flow passes through the centrifugal separator. The separated liquid drops are thrown from the side edges of the separator wipers to the apparatus wall and the flying liquid drops come into contact with the steam flow entering the separator, whereby secondary capture of the liquid becomes possible. It should be borne in mind that any increase of the separator height, the number of wipers and their spacing involves serious complications due to inadmissible added drag of the apparatus normally operating at a pressure of 1-5 mm Hg.
The apparatus is complicated and expensive to make because of the plurality perforations (thousands or tens of thousands) to be punctured in the depressions of the drum crimps for passing steam from the annular space between a drum and the casing into the inner space of a drum. Besides, the perforations are responsible for a certain amount of drops of liquid being carried away by the steam flow passing to the drum inner space.